As is well known, various processes and catalysts exist for the homopolymerization or copolymerization of olefins. Traditional Ziegler-Natta catalyst systems—a transition metal compound cocatalyzed by an aluminum alkyl—are capable of producing polyolefins having a high molecular weight but a broad molecular weight distribution. Traditional types of Ziegler-Natta catalysts have very high activities, and the polyolefins produced therewith have low catalyst residues and do not require a subsequent catalyst residue deashing treatment.
More recently a “metallocene” type catalyst system has been developed—one wherein the transition metal compound has cyclopentadienyl ring ligands, preferably at least two; such transition metal compound being referred to as a “metallocene”—which catalyzes the production of olefin monomers to polyolefins. Accordingly, metallocene compounds of the Group IV-B-metals, particularly bis(cyclopentadienyl) titanocenes and zirconocenes, have been utilized as the transition metal component in such “metallocene” containing catalyst system for the production of polyolefins and ethylene-α-olefin copolymers. When such metallocenes are cocatalyzed with an aluminum alkyl—as is the case with a traditional type Ziegler-Natta catalyst system—the catalytic activity of such metallocene catalyst system is generally too low to be of any commercial interest.
It has since become known that such metallocenes may be cocatalyzed with an alumoxane—rather than an aluminum alkyl—to provide a metallocene catalyst system of high activity for catalyzing the production of moderately high molecular weight polyolefins. Unfortunately, the amount of alumoxane cocatalyst required to provide the metallocene component with high activity is generally high, generally expressed as a molar ratio of Al to transition metal, hence a polyolefin produced from such metallocene-alumoxane catalyst may contain an undesirable amount of catalyst residue (or ash content, measured as the nonvolatile aluminum and transition metal content).
More recently, a new method of activating Group IV-B metallocene alkyl complexes was disclosed in European Patent Applications Nos. 277,003 and 277,004. The improved metallocene catalysts are prepared by combining at least two components. The first component is a bis(cyclopentadienyl) derivative of a Group IV-B metal containing at least one ligand which will combine with the cationic portion of the second component. The second component is an ion-exchange reagent comprising a cation which will irreversibly react with at least one ligand contained in said Group IV-B metal compound (first component) and a non-coordinating anion which is bulky, labile and stable. Suitable non-coordinating anions disclosed in these applications include: 1) anionic coordination complexes comprising a plurality of lipophilic radicals covalently coordinated to and shielding a central charge-bearing metal or metalloid core, and 2) anions comprising a plurality of boron atoms such as carboranes, metallocarboranes and boranes. Upon combination of the first and second components, the cation of the second component reacts with one of the ligands of the first component, thereby generating an ion-pair consisting of a Group IV-B metallocene cation with a formal coordination number of 3 and a valence of 4+ and the aforementioned non-coordinating anion. The disclosures are limited to catalyst systems derived from Group IV-B metallocene complexes containing at least two cyclopentadienyl ligands.
Aluminum alkyl-free olefin polymerization catalysts derived from transition metal complexes containing fewer than two cyclopentadienyl ligands have been relatively unexplored. John Bercaw reported [Organometallics, 1990, 9, 867] the synthesis of a monocyclopentadienyl scandium polymerization catalyst, [Me2Si(C5Me4)(N-But)ScH(PMe3)]2. This neutral Group III catalyst has a low activity and is very expensive due to the high cost of scandium metal. The need exists to provide a method of preparing highly active, versatile, aluminum alkyl-free olefin polymerization catalysts derived from monocyclopentadienyl ligand systems.